Disorders of perception, attention, and memory frequently accompany the major mental diseases. To begin to understand the neural mechanisms of these mental processes, we are recording the activity of neurons in the extrastriate cortex of monkeys engaged in tasks requiring visual discrimination, selective attention, an recognition memory. We found that selective attention gates visual processing by (1) filtering unwanted information from the receptive fields of extrastriate neurons, (2) sharpening neuronal selectivity in tasks requiring increased attention, and (3) enhancing neuronal responses when a stimulus must be stored in short- term memory. To identify the mechanisms by which cognitive state modulates cortical activity, we are examining both extrastriate neuronal activity and animal behavior in an attention-demanding task following chemical deactivation of subcortical structures that provide direct or indirect inputs to extrastriate cortex. These studies have identified two structures, the lateral pulvinar and the superior colliculus, that appear to be critical for the ability to focus attention on a single stimulus and ignore distracting stimuli.